Aluminum and aluminum alloys-heat treatment of aluminum and aluminum alloys(3)

Times strengthening

After solution treatment and quenching, hardening is achieved at room temperature (natural aging) or precipitation heat treatment (artificial aging). In some alloys, sufficient precipitation occurs within a few days at room temperature to produce a stable product with properties sufficient for many applications. These alloys are sometimes subjected to precipitation heat treatment to increase the strength and hardness of forged and cast alloys. Other alloys that have a slow precipitation reaction at room temperature are always precipitation heat treated before use. In some alloys, especially those of the 2xxx series, the cold working of freshly quenched material greatly increases its response to the later precipitation treatment. Mills use this phenomenon to produce higher mechanical properties by applying a controlled amount of rolling (plate and sheet) or stretching (extrusion, bar and plate). However, if higher characteristics are used in the design, reheating must be avoided.

The time for natural aging may range from about 5 days for 2xxx series alloys to about 30 days for other alloys. 6xxx and 7xxx series alloys have greatly reduced stability at room temperature, and have shown changes in mechanical properties over the years. For some alloys, by refrigerated at -18°C or lower, natural aging may be suppressed or delayed for several days. Before aging changes the properties of the material, it is common practice to complete forming, straightening and embossing. Traditional practice allows refrigeration alloy 2014-T4 rivets to maintain good driving characteristics.

Artificial aging or precipitation heat treatment is a long-term process at low temperature. The temperature ranges from 115-200°C and the time ranges from 5-48 hours. As with solution processing, precise temperature control and spatially varying temperatures are critical to this process, and the temperature should normally be kept within ±7°C.

Careful consideration should be given to the time of precipitation treatment-changes in temperature parameters. Longer time and higher temperature will result in larger particles or sediment. The goal is to select the cycle that produces the best precipitation size and distribution pattern. Unfortunately, the cycles required to maximize one property (such as tensile strength) are generally different from those required to maximize other properties, such as yield strength and corrosion resistance. Therefore, the cycles used represent a compromise that provides the best combination of performance.